1. Field of the Invention
This invention relates generally to chemical oxygen generators, and more particularly concerns an improved filter for removing chlorine and carbon monoxide in oxygen produced by chemical oxygen generators.
2. Description of Related Art
Chemical oxygen generators are typically used in situations requiring emergency supplemental oxygen, such as in aviation, during decompression, in mine rescue operations, in submarines, and in other similar settings. Chemical oxygen generating compositions based upon the decomposition of alkali metal chlorates or perchlorates have long been used as an emergency source of breathable oxygen, such as in passenger aircraft, for example. Oxygen for such purposes must be of suitably hih purity. For example, the requirements of SAE Aerospace Standard AS8010C are frequently applicable to oxygen used for breathing in aviation applications.
A typical chemical oxygen generating candle may have several layers with different compositions to obtain different reaction rates and flow rates which are desired at different stages during the period of operation. The candle typically has a generally cylindrical shape with a taper, with a recess at one end to hold an ignition pellet. The ignition pellet is ignited by firing a primer, and heat from the ignition pellet then ignites the reaction of the candle body, generating oxygen.
Chemical oxygen generators commonly utilize sodium chlorate, potassium perchlorate, and lithium perchlorate as sources of oxygen. Upon decomposition, the chlorate or perchlorate releases oxygen. In a typical chemical oxygen generator, a sodium chlorate candle is encased in a stainless steel canister, and oxygen is generated by decomposition of sodium chlorate in the presence of a commonly used fuel, such as iron powder, to provide extra heat to sustain the decomposition. Up to several hundred ppm chlorine gas is typically produced along with the oxygen, through side reactions and some organic contamination.
Iron powder typically contains 0.02% to 1% carbon that can also contaminate the oxygen released with up to 1,000 ppm of carbon monoxide. Above 710.degree. C., thermodynamic constraints also favor carbon monoxide formation over formation of CO.sub.2. Since iron is a very energetic fuel, and loading can be relatively high in some portions of the candle, temperatures in excess of 710.degree. C. can easily be reached. Even after oxygen evolution has ceased in those sections of the candle, temperatures typically continue to rise due to the oxidizing environment that is produced that can increase the extent of oxidation of iron. Thus, high levels of carbon monoxide in the oxygen produced by the initial stages of a candle fueled by carbon-containing metal powders such as iron are common, so that both chlorine gas and carbon monoxide must be removed to provide a safely breathable gas. The percussion primer, commonly used as an actuating means, contains organic compounds which can be a source of carbon monoxide. Electrical squibbs can also produce carbon monoxide. Thus, some carbon monoxide can be a contaminant of the liberated oxygen, even when steps are taken to reduce or eliminate carbon content in other materials used. Currently typically no more than 0.2 ppm chlorine and 15 to 50 ppm carbon monoxide is allowed in the oxygen provided for aviation.
In order to use iron powder as a fuel in an oxygen generator, it is economically preferable to utilize a filter to convert the carbon monoxide produced to the less toxic carbon dioxide. Granular soda lime, which commonly is a mixture of calcium oxide with sodium hydroxide or potassium hydroxide, has been used for the removal of carbon dioxide, water vapor, and chlorine gas from oxygen produced by chemical oxygen generators, but soda lime does not remove carbon monoxide. In addition, upon reacting with the residual moisture in the oxygen, soda lime has a tendency to become a soft, sticky, sludge-like material that can cause the oxygen generator to fail.
Activated carbon has traditionally been used to remove chlorine gas. However, since the carbon burns at about 300.degree. C. in oxygen, it is not appropriate to use activated carbon in a pure oxygen, high temperature environment.
A great majority of conventional chemical oxygen generators have either cast filters or granular bed filters with hopcalite, which is a mixture of manganese dioxide and copper oxide, as their active component. A cast filter is commonly made by mixing hopcalite and ceramic fiber or glass fiber in water or optionally in an alkaline solution. The slurry is then poured into a filter mold, and the excess solution is drained, optionally using suction to facilitate removal of this excess solution. The filter is then ejected from the mold and dried. Since this filter uses only hopcalite and ceramic fibers, it is effective for carbon monoxide removal, but is less effective for chlorine gas removal. Use of the optional alkaline solution may enhance chlorine removal. Cast filters are usually cast one at a time, making the process slow and expensive. Cast filters are also susceptible to mechanical damage that can result in filter failure, and can add up to 50 grams to the weight of an iron fueled oxygen generation system, which can be considerable disadvantages if the oxygen generation system is to be used on board aircraft.
Granular hopcalite bed filters are also used in some chemical oxygen generators for removing carbon monoxide, and are generally packed in a filter bed at the outlet end inside of the generators. The granules typically have a particle size between 10 and 20 mesh. However, such granular hopcalite bed filters are not very efficient, and more than 40 grams of hopcalite are needed to be used for each generator. Granular hopcalite bed filters commonly have some activity in removing chlorine gas, but the capacity of this type of filter for chlorine gas is quite low; when the level of chlorine gas produced by chemical oxygen generators is approximately 100 ppm, the chlorine gas will typically break through the filter in less than five minutes. This is not acceptable for aviation applications, for which chemical oxygen generators are required to supply oxygen for at least ten minutes. Granular hopcalite bed filters are also susceptible to damage from vibration, which can be a problem, since chemical oxygen generators in aircraft are frequently subjected to vibration. During vibration, the granules abrade against one another, so that the particle sizes of the granules are gradually reduced, and the filter bed becomes more tightly packed, settling to the bottom of the filter bed and creating channels that can lead to failure of the filter. In order to avoid the effects of abrasion, settling and channeling, the filter bed is usually loaded as several layers of hopcalite granules with ceramic fiber or glass fiber pads in between the hopcalite layers. However, it is difficult to pack the layers of hopcalite evenly inside the filter compartments, and the assembly process is typically slow and tedious. The filter pads in between the hopcalite layers further increase the overall weight of the filter.
Granular hopcalite coated with sodium hydroxide, and particularly the fraction of the granular material having particle sizes between 10 and 20 mesh, has been used to make filters for chlorine removal. However, such filters are heavy and difficult to assemble, with three layers of hopcalite being used with fiber pads in between the layers, and the hopcalite alone weighing more than 40 grams. Furthermore, sodium hydroxide is deliquescent, absorbing moisture rapidly, so that the filter material needs to be isolated from the ambient air during manufacture of the filter, increasing manufacturing difficulty and costs. In addition, sodium hydroxide can form a sort of glaze or coating which can reduce surface area and greatly interfere with the ability of the filter to remove carbon monoxide and chlorine gas.
It is therefore desirable to provide a filter material that is more effective in removing chlorine and carbon monoxide, permitting less of the material to be used in making the filters so that the filters can be made lighter and easier to manufacture than conventional filters. The present invention meets these needs.